Sputter deposition and plasma modification of tungsten alloys for nuclear fusion applications

Abstract

The ITER reactor aims to be first thermonuclear fusion device to demonstrate net fusion power. Critical plasma-facing components in the reactor will be made of tungsten, but helium plasma irradiation is known to embrittle tungsten metal. A future reactor will therefore need an alternative material to extend the lifetime of the components. This thesis investigated the use of tungsten alloy films as a plasma-facing material for a future reactor. Tantalum and chromium were chosen as the alloying elements, and 60 − 100 nm films were manufactured using magnetron sputter deposition at a variety of alloy concentrations. Tungsten alloy films are known to deposit in an undesirable A15 crystal phase. The films were heat treated at 650◦C for 1 hour, 2 hours and 4 hours to induce and investigate the phase transition from A15 to BCC structure. X-ray diffraction patterns confirmed the presence of an A15 structure prior to annealing. The A15 phase was very stable in comparison with past results, requiring over 2 hours at 650◦C to transition to BCC. This was speculated to be linked to oxygen trapped in the films during deposition. The phase transition was slower in alloyed films than a pure tungsten control, with the retarding effect of tantalum stronger than chromium, which suggested the alloying atoms further stabilised the deposited A15 structure. Resistivity of the films saw a reduction with annealing time characteristic of the phase transition. The annealed alloy films were then exposed to helium plasma in the Magnetised Plasma Interaction Experiment at 300◦C, 500◦C and 800◦C. Surface morphology changes were tracked with secondary electron imaging. At 800◦C, surface pits were seen, which were larger in a pure tungsten film than in low concentration alloy films. Advanced surface roughening was observed in the chromium films, while a high concentration tantalum film had no surface damage at all. X-ray diffraction patterns confirmed these trends, and showed an increase in microstrain with the surface roughening in the chromium alloys. These results suggest W-Ta alloy films may have superior irradiation resistance than W-Cr alloy films, but more work is needed to confirm the trends in alloy concentration.